Work perfectly do not forget:
In your Firefox media keys may be disabled. To enable the functionality:
1. Navigate to about:config from your address bar
2. If there is a warning message just confirm that you understand the risks
3. Search for media.hardwaremediakeys.enabled and set to TRUE
3. Search for dom.media.mediasession.enabled and set to TRUE
4. Restart your browser

:)

its vary nice

This add-on is still working on my Firefox now(Feb~Dec-2023).
On some version of Firefox, You must maybe settings(Option or "about:config").
Please read more about this add-on page.
After add this Extension, YouTube Videos became not pause. Thanks!

2023/12/2 P.S.
If playlist still pauses, some settings may be incorrect.
First make sure latest version is "0.9.2".
In some version of Firefox, turning on the permission switches of accessing "youtube.com" for may work (Manage Extensions).

Works like a charm, Thank you for making this!